The present invention relates generally to a process and an apparatus for the production of hollow bodies from thermoplastic material, the wall of which is in the form of a laminate, by means of extrusion blow molding.
In a process for producing a hollow body from thermoplastic material with a wall in the form of a laminate comprising a plurality of interconnected layers of at least first and second plastic materials of different natures, tubular preforms, the wall of which has a suitable number of layers, are first produced in a batch-wise manner using an extrusion unit comprising at least first and second extruders and a common extrusion head which is provided with an annular storage space or chamber for the laminate, an annular piston for emptying the storage space or chamber and an extrusion opening through which the preforms are extruded. All the flows of material which pass into the extrusion head and from which the individual layers of the laminate structure are formed are passed through the annular piston and are brought therein into the cross-sectional shape of a ring. It will be seen therefore that all the flows of material are already combined together to form the final laminate from which the wall of each hollow body is to be produced, within the annular piston of the extrusion unit. Thereupon, the final laminate which is formed in that way flows towards the storage chamber or space through an annular duct portion in the annular piston, that duct portion being enlarged in a funnel-like configuration in the direction of flow of the material. In the region of the annular piston in which the individual layers are combined to form the final laminate, the latter is of a thickness, that is to say a radial dimension, which is substantially less than the radial dimension or width of the storage space or chamber. That difference can be bridged by the conically enlarging duct portion in the annular piston so that at the mouth opening of the duct the final laminate is of approximately the same radial dimension as the storage chamber.
It will be noted at this point however that that operating procedure which involves the individual flows of material being brought together to form the final laminate structure at a location at which the laminate structure is of only small thickness in comparison with the width of the storage chamber in order then to be adjusted within the piston to a thickness which approximately corresponds to the width of the storage chamber presupposes the use of an extrusion head which must be very long as the piston must be of a corresponding length in order for the process to be performed in that way. In addition, a laminate-formation location of small radial dimension, that is to say with the laminate being of small thickness at the moment at which it is formed, means that the individual flows of material which are combined together at that location to form the laminate must be moving at a correspondingly high rate, with the result that differences in flow rate as between the individual flows of material, at the moment at which the laminate is formed therefrom, are of corresponding magnitude, for the relative thicknesses of the respective layers of the laminate structure derive from the flows of material occurring under suitable conditions in that respect, that is to say, when there are substantial differences in thickness between the individual layers, greatly different flow rates are involved.